The First Inhabitants
The original inhabitants of Earth were Prokaryotes. They are single-celled organisms divided into two kingdoms - Bacteria and Archaea. They don't come much tougher than Prokaryotes. And having survived the most hostile of environments the Earth has ever experienced in its history, they are still found today just about everywhere and in the harshest of environments. They have existed for almost 4 billion years. And we must remember that at that time there was no ozone to block the sun's lethal ultraviolet radiation and the atmosphere was full of noxious chemicals, released mainly from volcanic activity and the almost continuous bombardment of Earth from the space debris left over after the formation of the solar system.

Today they thrive in boiling hot geothermal springs, deep below the frozen Antarctic, miles deep in the highly pressurised Earth's crust, and even floating high up in the atmosphere (did they drift from space into the Earth's atmosphere?). Certainly the vacuum of space doesn't seem to pose a problem to Bacteria. We saw when Apollo 12 recovered a camera from the moon in November 1969, that despite having been left there 30 months earlier, Bacteria on the camera were still alive. It was discovered that they had been on the camera before launch which meant they had survived 30 months in a vacuum. Not to mention enduring the massive temperature swings averaging over 100°C to -150°C.

Could Bacteria be immortal? This is what many Scientists are now asking as we begin to learn more about them. Bacteria have been revived that were last active up to 250 million years ago. They appear to possess the ability to remain in suspended animation for long periods of time.

The oldest macrofossils disovered are stromatolites. Formed by living microorganisms along seacoasts and shallow seas, which trapped and bound sand grains together, and thereby constructing these layered mound-shaped deposits. This record of the evolution of early life dates back to 3.5 billion years ago and the structures of the stromatolites suggests that there was much biological activity at that time and also provides evidence that these microorganisms sought light and were therefore photosynthetic.

Most of the history of life on Earth involved the evolution of single-celled microrganisms which dates back 3.5 billion years. We can only conclusively identify multicelled (organism made of different cell types) fossils in rocks much younger, about 600 million years. This was the Precambrian period and the first multi-celled animals were called Metazoa. These are soft-bodied animals and so are very difficult to detect in the fossil record. Perhaps multi-celled life did start much earlier but were just not prone to fossilization.

Prokaryotes do not have a nucleus or other membrane bound organelles (internal structures). They are single-celled organism of which Bacteria are the most prolific examples. And like all cells, bacteria contain the genetic material DNA, but it's not confined to a nucleus. They first existed about 3.5 billion years ago.

It was perhaps 3 billion years ago that life first began to shape the Earth's atmosphere using the process of photosynthesis on cyanobacteria which essentially is the conversion of solar energy to chemical energy, and one of the most important chemical reactions to take place within cells. It's the process we are familar with today where modern plants (and some bacteria) use the energy from sunlight, as well as carbon dioxide and water to produce sugar (which is converted into the fuel for all living cells) as well as oxygen. As photoynthesising organisms began to flourish 2-3 billion years ago, oxygen began to build up in the atmosphere. Eventually, in this newly evolving atmosphere multicellular organisms (eukaryotes) evolved that could 'breathe' the ever increasing amounts of oxygen.

All cells existing prior to about 1 billion years ago were prokaryotes. Eukaryotes, which have a cell nucleus and organelles only appeared after that.

Today we know that prokaryotes do cluster around vents of superheated water in mid-ocean ridges as well as at hot springs on land. Most life on Earth depends on photosynthesis, where life makes energy from sunlight. But around the vents, life exploits the energy released when the chemical rich hot water mixes with cold sea water in a process called chemosynthesis.

Eukaryotes began to appear less than 1 billion years ago, surely aided by the accumulation of oxygen in the atmosphere which had important consequences such as setting the stage for the advent of aerobic respiration (which produces energy in the cells necessary for the various life processes - converts stored food into energy). Up to that stage anaerobic respiration dominated, but aerobic respiration had an advantage in that many times more energy was generated which benefited organisms using aerobic respiration.

The accumulation of oxygen was also vital for the crucial ozone layer which shielded life from the harmful ultraviolet radiation.

The Eukaryotes cells were more structurally organised than the Prokayotes, and had complex systems of organelles and membranes. The genetic material is arranged in chromosomes within a nucleus. They contain organelles such as mitochondria - which generates chemical energy for the cell and chlorplasts - which are crucial to photosynthesis in plants. Eukaryotes eventually began to 'experiment' with multicelled body structures.

Animals and plants most familiar to us were evolutionary latecomers evolving only in the last 550 million years. Marine invertebrates (such as shell-making ammonites) appeared first, then fish (500 mya), amphibians (360 mya), reptiles (300 mya), mammals (200 mya) and birds (150 mya). Land plant communities also evolved in complexity from relatively ancient clubmosses and ferns, to the more recent gymnosperms (eg, conifers) and angiosperms (flowering plants).

To-date we do not know precisely how or when (or why) life began. How did we get from non-life to life? From non-living chemicals to simple organisms? From something that could not reproduce to something that could? However, we do have a good knowledge of the early Earth and some of the essential requirements to produce life. We do know early Earth was dominated by volcanoes, a lifeless ocean and turbulant atmosphere with chemically active clouds penetrated by lightning and solar radiation. And the more accurate a picture we can form of the early Earth then the greater our understanding of the evolution of life during that time period will be.